chore: drop dead _qid helper and surface ocr_confidence on SearchHit

- app/indexing/qdrant_client.py: remove the identity-only _qid()
  helper and pass chunk_id straight to PointStruct (Qdrant accepts
  the UUID string directly).
- services/types.ts: SearchHit gets an explicit, optional
  ocr_confidence field so consumers can type the value instead of
  casting through metadata.
- widgets/SearchResultCard.tsx: replaces the
  (hit.metadata as { ocr_confidence? }) cast with the new field. No
  behavior change when the backend omits it.

tsc --noEmit: clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/services/types.ts

@@ -47,6 +47,7 @@ export interface SearchHit {
   citation: Citation;
   quality_flags: QualityFlags;
   metadata: Record<string, unknown>;
+  ocr_confidence?: number | null;
 }
 
 export interface SearchResponse {

frontend/src/widgets/SearchResultCard.tsx

@@ -20,9 +20,7 @@ interface Props {
 }
 
 export function SearchResultCard({ hit, query, active, onSelect, reranked }: Props) {
-  const ocrConf =
-    (hit.metadata as { ocr_confidence?: number })?.ocr_confidence ??
-    null;
+  const ocrConf = hit.ocr_confidence ?? null;
   return (
     <motion.button
       layout